In a telephone network which employs analog transmission over 2-wire circuits, echo is introduced at the point of conversion between 4-wire and 2-wire circuits--typically referred to as the "hybrid". This echo results from an imperfect impedance match between the 4-wire and 2-wire circuits. Proper control of impedance matching, however, limits the echo to an allowable level.
The subjective effect of this echo is quite acceptable. Indeed, a controlled amount of near-end echo, known as sidetone, is desirable to prevent a telephone from sounding "dead". When transmission delay is introduced into the voice path, however, the effects of echo become much more objectionable and may ultimately render a circuit unusable. This situation can occur in the public network if multiple transatlantic hops utilizing geo-stationary satellites are connected in tandem. In order to mitigate this effect, long distance circuits in the public network are often equipped with echo suppressers which attenuate the strength of unwanted echo signals.
Local telephone networks and customer premise equipment are not equipped with echo suppressers, since local connections made by conventional space/time division switching means can never encounter the long transmission delays that would produce unacceptable echoes.
However, it is becoming more common to provide voice services over packet switched networks using technologies such as Asynchronous Transfer Mode, Ethernet, isoEthernet and TCP/IP. Control over transmission delay is less secure under packet switched technologies than with space/time division switching and in some cases may not be provided at all. This migration from space/time division switching to packet switching creates a requirement for echo suppression to be provided in local switching and transmission equipment.
There exists another situation in which echo can be introduced to a local telephone network. This occurs when speakerphones are utilized. Sounds originating from a far-end talker are broadcast into the room occupied by the speakerphone user. Some of this sound is reflected off objects in the room and the walls of the room itself, and picked up by the microphone attached to the speakerphone. This reflected sound is transmitted back to the far-end talker and experienced as echo. This situation occurs even when 4-wire circuits are used throughout the connection.
There is, therefore, a need for a method and apparatus for controlling echo at its point of introduction to a local telephone network, in a practical and cost-effective manner.